Social complexity, life-history and lineage inuence the molecular basis of caste in a major transition in evolution Christopher Wyatt ( [email protected] ) University College London Michael Bentley University College London Daisy Taylor University College London Emeline Favreau University College London https://orcid.org/0000-0002-0713-500X Ryan Brock University of East Anglia https://orcid.org/0000-0003-2977-1370 Benjamin Taylor University College London Emily Bell School of Biological Sciences, University of Bristol https://orcid.org/0000-0002-7309-5456 Ellouise Leadbeater Royal Holloway University of London https://orcid.org/0000-0002-4029-7254 Seirian Sumner UCL https://orcid.org/0000-0003-0213-2018 Article Keywords: Superorganismality, Castes, Wasps, Transcriptomics Posted Date: August 27th, 2021 DOI: https://doi.org/10.21203/rs.3.rs-835604/v1 License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License 1 Social complexity, life-history and lineage influence the molecular basis of caste in a major 2 transition in evolution 3 4 Christopher D. R. Wyatt1*, Michael Bentley1,†, Daisy Taylor1,2,†, Emeline Favreau1, 5 Ryan E. Brock2,3, Benjamin A. Taylor1, Emily Bell2, Ellouise Leadbeater4 & Seirian 6 Sumner1* 7 8 1 Centre for Biodiversity and Environment Research, University College London, 9 London, UK. 10 2 School of Biological Sciences, University of Bristol, United Kingdom, BS8 1TQ. 11 3 School of Biological Sciences, University of East Anglia, Norwich Research Park, 12 Norwich, Norfolk, NR4 7TJ, UK 13 4 Department of Biological Sciences, Royal Holloway University of London, Egham, 14 UK. 15 * Corresponding authors 16 † Equal contribution 17 18 Christopher Douglas Robert Wyatt 19 Centre for Biodiversity and Environment Research, 20 University College London, London, UK. 21 E-mail: [email protected]; Phone +447307390637 22 23 Seirian Sumner 24 Centre for Biodiversity and Environment Research, 25 University College London, London, UK. 26 E-mail: [email protected] 27 28 Key words: Superorganismality, Castes, Wasps, Transcriptomics 1 29 Abstract 30 Major evolutionary transitions describe how biological complexity arises; e.g. in the 31 evolution of complex multicellular bodies, and superorganismal insect societies. 32 Such transitions involve the evolution of division of labour, e.g. among queen and 33 worker castes in insect societies. A key mechanistic hypothesis for the evolution of 34 division of labour is that a shared set of genes co-opted from a common solitary 35 ancestral ground plan - a so-called genetic toolkit for sociality - regulates insect 36 castes across different levels of social complexity. The vespid wasps represent an 37 excellent system in which to test this. Here, using conventional and machine learning 38 analyses of brain transcriptome data from nine species of vespid wasps, we find 39 evidence of a shared genetic toolkit across species representing different levels of 40 social complexity, with a large suite of genes classifying castes correctly across 41 species. However, we also found evidence of additional fine-scale differences in 42 predictive gene sets, functional enrichment and rates of gene evolution that were 43 related to level of social complexity, but also life-history traits (e.g. mode of colony 44 founding). Thus, there appear to be shifts in the gene networks regulating social 45 behaviour and rates of gene evolution that are influenced by innovations in both 46 social complexity and life-history. These results suggest that the concept of a shared 47 genetic toolkit for sociality may be too simplistic to fully describe the process of the 48 major transition to sociality, even within a single lineage. Diversity in lineage, social 49 complexity and life-history traits must be taken into account in the quest to uncover 50 the molecular bases of the major transition to sociality. 51 2 52 Main 53 The major evolutionary transitions span all levels of biological organisation, 54 facilitating the evolution of life’s complexity on earth via cooperation between single 55 entities (e.g. genes in a genome, cells in a multicellular body, insects in a colony), 56 generating fitness benefits beyond those attainable by a comparable number of 57 isolated individuals1. The evolution of sociality is one of the major transitions and is 58 of general relevance across many levels of biological organisation from genes 59 assembled into genomes, single-cells into multi-cellular entities, and insects 60 cooperating in superorganismal societies. The best-studied examples of sociality are 61 in the hymenopteran insects (bees, wasps and ants) - a group of over 17,000 62 species, exhibiting different forms of sociality across the transition from simple 63 sociality (with small societies where all group members are able to reproduce and 64 switch roles in response to opportunity), through to complex societies (consisting of 65 thousands of individuals, each committed during development to a specific 66 cooperative role and working for a shared reproductive outcome within the higher- 67 level ‘individual’ of the colony, known as the ‘superorganism’2). Recent analyses of 68 the molecular mechanisms of insect sociality have revealed how conserved suites of 69 genes, networks and functions are shared among independent evolutionary events 70 of insect superorganismality3–8. An outstanding question is whether there are fine- 71 scale differences in the genetic mechanisms operating at different levels of 72 complexity across the spectrum of the major transition – from the simplest to most 73 complex societies. 74 A key step in the evolution of sociality is the emergence of a reproductive 75 division of labour, where some individuals commit to reproductive or non- 3 76 reproductive roles, known as queens and workers respectively in the case of insect 77 societies. An overarching mechanistic hypothesis for social evolution is that the 78 repertoire of behaviours typically exhibited in the life cycle of the solitary ancestor of 79 social species were uncoupled to produce a division of labour among group 80 members with individuals specialising in either the reproductive (‘queen’) or 81 provisioning (‘worker’) phases of the solitary ancestor9. Such phenotypic decoupling 82 implies that there will be a conserved mechanistic toolkit that regulates queen and 83 worker phenotypes in species representing different levels of social complexity 84 across the spectrum of the major transition (reviewed in10). An alternative to the 85 shared toolkit hypothesis is that the molecular processes regulating social 86 behaviours in non-superorganismal societies (where caste remains flexible, and 87 selection acts primarily on individuals) differ fundamentally from those processes that 88 regulate social behaviours in superorganismal societies 11,12. Phenotypic innovations 89 across the animal kingdom have been linked to genomic evolution: taxonomically- 90 restricted genes13–16, rapid evolution of proteins17,18 and regulatory elements17,19 91 been found in most lineages of social insects20. Indeed, some recent studies have 92 suggested that the processes regulating different levels of social complexity may be 93 different8,17,19,21. The innovations in social complexity and the shift in the unit of 94 selection (from individual- to group-level22) that occur in a major transition may 95 therefore be accompanied by shifts in genomic processes and evolution, throwing 96 into question whether the concept of a universal conserved genomic toolkit that 97 regulates social behaviours across the spectrum of the major transition may be too 98 simplistic23. The roles of conserved and novel processes are not necessarily 99 mutually exclusive; novel processes may coincide with phenotypic innovations, whilst 4 100 conserved mechanisms may regulate core processes at all stages of social 101 evolution. 102 Until recently, the data available to test these hypotheses were largely limited 103 to species that represent either the most complex – superorganismal - forms of 104 sociality (e.g. most ants, honeybees24), or the simplest forms of social complexity as 105 non-superorganisms (e.g. Polistes wasps7,25–27 and incipiently social bees28–31), the 106 latter of which likely represent the first stages in the major transition. Recent studies 107 have identified core gene sets underlying caste-differentiated brain gene expression 108 across a range of ants5 and bees8; ants lack ancestrally non-superorganismal 109 representatives and so may not be informative for studying simpler societies and, 110 putatively, the process of social evolution32. Analyses of molecular evolution across 111 two independently evolved lineages of sociality in bees showed increased levels of 112 regulatory complexity with social complexity. These insights highlight the need for a 113 fine-scale dissection of the molecular processes regulating social behaviour for 114 species representing different forms of social complexity8, but also the need to 115 consider the influence of life-history traits on gene evolution, as a selective process 116 that is independent of complexity per se. 117 A promising group for unpicking the molecular basis of social complexity are 118 the vespid wasps33, which exhibit the full diversity of social complexity among some 119 1,100 species, as well as diversity in life-history traits such as mode of colony 120 founding. Aculeate wasps are of paramount importance in studying the evolution of 121 sociality